1. Field of the Invention
The present invention relates to an optical position measuring arrangement for detecting the position of two objects which are movable in relation to each other in at least one measuring direction, and have a scale connected with one of the two objects, and a scanning unit connected with the other one of the two objects.
2. Discussion of Related Art
An optical position measuring arrangement in accordance with the species is known from EP 513 427 B1 of Applicant. It is suitable for detecting the position of two objects which are movable in relation to each other in at least one measuring direction. For this purpose, the known position measuring arrangement includes a scale, which is connected with one of the two objects. The scale has an incremental graduation extending in the measuring direction, as well as at least one reference marking at a reference position. Here, the reference marking includes a structure with a graduation period which can change locally, i.e. it is made of a structure comprising a plurality of different graduation periods. Such structures are also designated as so-called chirped graduation structures or chirped gratings. The position measuring arrangement furthermore has a scanning unit, which is connected with the other one of the two objects and has scanning elements which are used for generating at least one displacement-dependent incremental signal, as well as at least one reference signal at a reference position by optical scanning of the incremental graduation and the reference marking along the measuring track.
The position measuring arrangement known from EP 513 427 B1 is based on a so-called interferential scanning principle. Here, the displacement-dependent scanning signals in the form of the incremental and reference signals are obtained from the constructive and destructive superposition of several partial light beams which, in case of a relative movement of the scale and the scanning unit, undergo displacement-dependent phase shifts. In this way it is possible to obtain highly resolved position information in regard to the relative position of the two objects.
In such systems the resultant pulse width of the reference signal is customarily selected to be such that it corresponds to the resulting signal period of the incremental graduation. To assure this it is necessary to select the locally varying graduation periods TPREF of the reference marking to be in a definite, or fixed, relationship with the graduation period TPINC of the incremental graduation. In actual use, the selected graduation periods TPREF of the reference marking embodied as a chirped grating typically extend in the range between 1.5*TPINC and 5*TPINC. Besides the above mentioned publication, reference is furthermore made in this connection to DE 197 48 802 A1 of Applicant, which discloses such dimensioning rules for chirped reference markings. This means that the graduation periods TPREF of the chirped reference marking are customarily clearly greater than the graduation periods TPINC of the incremental graduation. In regard to the optical effects on the light beams striking it, this means that the resultant diffraction or deflection angles of the light beams for reference signal generation are clearly narrower than the resultant diffraction angles of the light beams for incremental signal generation. However, narrower deflection angles of the partial light beams employed for reference signal generation entail problems in connection with certain scanning principles. Thus, there are optical position measurement devices, such as disclosed in DE 101 44 659 A1, which require a spatial separation of the partial light beams striking the scale and deflected by it. In the case of very narrow deflection angles of the partial light beam employed for the reference signal generation, this can only be assured if a very large scanning gap between the scale and the scanning unit is selected. In turn, a very large scanning gap is disadvantageous with respect to the installation tolerance of the position measuring device. Moreover, a large scanning gap requires an elaborate collimation of the light sources in the scanning unit.